7.4 Absorption and stripping

Absorption and stripping are key mass transfer operations in chemical engineering. They involve the exchange of components between gas and liquid phases. These processes are crucial for separating mixtures, purifying gases, and recovering valuable materials in various industries.

Understanding absorption and stripping is essential for designing efficient separation systems. We'll explore the principles behind these processes, column designs, optimal operating conditions, and performance evaluation methods. This knowledge is vital for creating effective and economical separation processes in chemical engineering.

Principles and applications of absorption and stripping

Fundamentals of absorption and stripping processes

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• Absorption is a mass transfer operation where a soluble component of a gas mixture is dissolved in a liquid solvent
• Stripping is the reverse process where a dissolved component is removed from a liquid solution by contact with a gas
• The principles of absorption and stripping are based on the solubility of gases in liquids governed by factors such as temperature, pressure, and the chemical nature of the gas and liquid
• Absorption is driven by the concentration gradient between the gas and liquid phases, with the solute transferring from the gas phase to the liquid phase until equilibrium is reached
  • The concentration gradient is the difference in the concentration of the solute between the gas and liquid phases
• Stripping is driven by the concentration gradient in the opposite direction, with the solute transferring from the liquid phase to the gas phase

Applications of absorption and stripping processes

• Absorption is used for the removal of acidic gases from natural gas (CO2, H2S)
• Absorption is used for the recovery of valuable components from process gas streams (ammonia, hydrocarbons)
• Absorption is used for the purification of air or other gases (removal of contaminants, moisture)
• Stripping is used for the removal of volatile organic compounds (VOCs) from wastewater (benzene, toluene)
• Stripping is used for the regeneration of solvents (removal of dissolved gases, impurities)
• Stripping is used for the deaeration of liquids (removal of dissolved oxygen, carbon dioxide)

Design of absorption and stripping columns

Types of absorption and stripping columns

• Absorption and stripping columns are typically designed as packed towers or tray columns
• The choice of column type depends on factors such as the gas and liquid flow rates, the required separation efficiency, and the pressure drop
• Packed towers use a bed of packing material to provide a large interfacial area for mass transfer between the gas and liquid phases
  • Common packing materials include Raschig rings, Pall rings, or structured packing
  • The liquid flows downward through the packing, while the gas flows upward countercurrently
• Tray columns use a series of horizontal trays or plates to create stages for gas-liquid contact
  • The liquid flows across each tray and then down to the next tray through downcomers
  • The gas rises through openings in the trays, bubbling through the liquid on each tray

Design considerations and analysis methods

• The design of absorption and stripping columns involves determining the column diameter, height, and internals (packing or trays) based on the required separation efficiency, gas and liquid flow rates, and the properties of the gas and liquid
• Key design parameters include the gas and liquid flow rates, the inlet and outlet concentrations, the equilibrium curve, and the mass transfer coefficients
• The column diameter is determined by the gas and liquid flow rates and the allowable pressure drop
• The column height is determined by the number of transfer units (NTU) and the height of a transfer unit (HTU)
• Analysis of absorption and stripping columns involves using mass transfer and equilibrium relationships to predict the concentration profiles, separation efficiency, and pressure drop
  • The overall mass transfer coefficient quantifies the rate of mass transfer between the gas and liquid phases
  • The equilibrium curve relates the concentration of the solute in the gas and liquid phases at equilibrium
  • The operating line represents the actual concentration profile in the column and is determined by the gas and liquid flow rates and the inlet and outlet concentrations

Optimal operating conditions for absorption and stripping

Factors affecting the optimal operating conditions

• The optimal operating conditions for absorption and stripping systems depend on factors such as the desired separation efficiency, the properties of the gas and liquid, and the economic constraints
• Temperature and pressure are critical operating variables that affect the solubility of gases in liquids and the driving force for mass transfer
  • Higher temperatures generally decrease gas solubility, while higher pressures increase gas solubility
• The liquid-to-gas ratio (L/G) is another important operating parameter that influences the separation efficiency and the size of the equipment
  • A higher L/G ratio improves separation efficiency but increases the liquid flow rate and the size of the column
• The choice of solvent is crucial for absorption processes, as it determines the solubility of the gas, the selectivity of the separation, and the ease of solvent regeneration
  • Common solvents include water, alkanolamines (MEA, DEA), and physical solvents (Selexol, Rectisol)

Optimization methods and tools

• The optimal operating conditions are often determined through process simulations, pilot-scale experiments, or optimization studies that consider the trade-offs between separation efficiency, energy consumption, and capital and operating costs
• Process simulations using software such as Aspen Plus, HYSYS, or ProMax can model the absorption or stripping process and predict the performance under different operating conditions
• Pilot-scale experiments can validate the simulation results and provide data for scale-up and design
• Optimization studies can use mathematical models and optimization algorithms to determine the optimal operating conditions based on specified objectives and constraints
  • Common optimization objectives include minimizing energy consumption, maximizing product purity, or minimizing total cost
  • Constraints may include the maximum allowable pressure drop, the minimum required separation efficiency, or the available equipment sizes

Performance evaluation of absorption vs stripping processes

Key performance parameters

• The performance of absorption and stripping processes can be evaluated using various key parameters that quantify the extent of mass transfer and the separation efficiency
• The number of transfer units (NTU) is a dimensionless parameter that represents the difficulty of the separation and the size of the equipment required
  • A higher NTU indicates a more difficult separation and a larger column
  • NTU is defined as the ratio of the overall mass transfer coefficient to the gas or liquid flow rate, multiplied by the interfacial area
• The height of a transfer unit (HTU) is the column height required to achieve one transfer unit and is used to determine the total column height
• The separation factor (α) is a measure of the relative volatility of the components being separated and indicates the ease of separation
  • A higher separation factor means an easier separation and a smaller column
  • The separation factor is defined as the ratio of the equilibrium constants (K-values) of the components being separated
• The absorption factor (A) is a dimensionless parameter that relates the liquid and gas flow rates to the equilibrium curve
  • It determines the minimum liquid-to-gas ratio required for a given separation
• The stripping factor (S) is the analogous parameter for stripping processes and relates the gas and liquid flow rates to the equilibrium curve

Performance evaluation and troubleshooting

• Other performance indicators include the removal efficiency, the purity of the product streams, the pressure drop, and the energy consumption
• These parameters can be used to compare different design options, optimize the operating conditions, and troubleshoot performance issues
• For example, a low removal efficiency may indicate insufficient column height, inadequate liquid or gas flow rates, or poor mass transfer
• A high pressure drop may suggest fouling of the packing or trays, excessive gas or liquid flow rates, or an undersized column diameter
• High energy consumption may result from excessive liquid or gas flow rates, high reboiler or condenser duties, or inefficient heat integration
• Troubleshooting performance issues involves analyzing the process data, comparing the actual performance with the design values, and identifying the root causes of the problems
  • This may require adjusting the operating conditions, cleaning or replacing the internals, or modifying the process design
  • Process simulations and pilot-scale experiments can help in identifying the optimal solutions and validating the proposed changes